The present invention is directed to a system and method for modulating data on an optical signal for transmission in an optical network.
Fiber-optic communication networks are experiencing rapidly increasing deployment. Especially rapid is the growth of network segments that carry multi-gigabit digital data on multiple wavelengths over a single fiber strand. Such segments are predominantly comprised of point-to-point fiber-optic links. Data-modulated optical signals originated at one end propagate through the fiber medium to the opposite end.
The dominant data modulation format in optical systems has been non-return to zero on-off keying (NRZ-OOK). In an NRZ-OOK format a binary xe2x80x98onexe2x80x99 is represented by light being xe2x80x98onxe2x80x99 and a binary xe2x80x98zeroxe2x80x99 by light being xe2x80x98offxe2x80x99. This format exhibits good spectral efficiency in multi-wavelength systems, sufficient distance capability, and straightforward implementation.
When longer transmission distances are required, such as in submarine and long-haul terrestrial fiber-optic links, the NRZ-OOK format is typically modified by returning the xe2x80x98onexe2x80x99 level to xe2x80x98zeroxe2x80x99 within each bit period (RZ-OOK), and possibly by adding some amount of optical phase modulation to each bit. This modulation format shows an enhanced distance reach, at the expense of more complicated components and reduced spectral efficiency.
The industry has explored other modulation formats that would allow for an increase in spectral efficiency by permitting denser packing of wavelength channels. While this does typically reduce transmission distance before electrical regeneration is required, many terrestrial links are already length-limited by network design constraints. Also, one of the fundamental limitations to this approach is the occurrence of direct cross talk between adjacent channels due to their spectral characteristics and to the finite rejection of selector filters.
One format attempting to reduce spectral bandwidth is known as multi-level (M-level) amplitude-shift keying (ASK) modulation. This produces optical spectrum compression by a factor of approximately log2 (M). The drawback is a rather severe degradation in susceptibility to noise produced by optical amplifiers, to the point of imposing unachievable optical noise characteristics. This method also requires receivers with multiple decision levels, which is a difficult accomplishment at gigabit bit rates.
Phase-shaped binary transmission (PSBT) is another approach to improving spectral efficiency. PSBT leaves some residual optical power in the xe2x80x98zeroxe2x80x99 bit, but imposes a xe2x80x98xcfx80xe2x80x99 phase shift at the center of this bit. This reduces optical bandwidth by a factor of about 2, and increases spectral efficiency by a factor of about 1.5 compared to conventional OOK systems. The PSBT format also provides for an improved tolerance to dispersion. The tradeoff, however, is that PSBT uses a transmitter that typically requires radio frequency (RF) signal processing. Also, PSBT requires signal matching to both arms of a Mach-Zehnder modulator.
Frequency-division multiplexing is a technique commonly used in the cable industry for transmission of multiple Megahertz-range data signals. Each Megahertz-range data stream is modulated onto an independent RF carrier and combined with other such signals in the RF domain. The composite signal then modulates an optical carrier. Such system can achieve very high spectral efficiency, but, as with the multi-level modulation, the performance penalties make it impractical for long-distance gigabit transport.
Accordingly, there is a need in the art for a system and method of modulating an optical signal that provides high channel density, i.e. spectral efficiency, and excellent signal transmission performance in multi-wavelength optical communication systems.
The present invention is organized about the concept of achieving, among other benefits, high spectral efficiency through use of a combination of phase and amplitude modulation. In particular, an optical device consistent with the invention includes a plurality of phase modulators, each of which is configured to provide an associated phase modulated optical signal; and a plurality of optical filters. Each of the filters receives an associated one of the optical signals and is configured to provide an amplitude modulated optical signal as an optical filter output signal.
An optical communication system consistent with the invention may include a plurality of phase modulators, each of which is configured to provide an associated phase modulated optical signal representing an associated data signal; and a plurality of transmitting optical filters. Each of the transmitting filters receives an associated one of the optical signals and amplitude modulates the signal as a transmitting filter output signal. At least one optical combiner receives a plurality of the transmitting filter output signals and transmits the transmitting filter output signals on an optical signal path as an aggregate signal. Each of a plurality of receiving optical filters receives at least a portion of the aggregate signal. Each filter is configured to provide an output signal comprising an associated one of the transmitting filter output signals.
A method of modulating a plurality of optical wavelengths in a wavelength division multiplexed optical communication system consistent with the invention includes: phase modulating each wavelength based on an associated data signal to provide an associated phase modulated signal; and coupling each phase modulated signal to an associated optical filter configured to provide an amplitude modulated optical signal based on an amplitude response of the filter.